The prior art teaches a variety of formulations and methods for making polymer thick-film resistors. Examples of such prior art are disclosed in U.S. Pat. Nos. 4,021,277, 4,368,252, 4,540,463, 4,865,873, 4,870,746, 5,039,570, 5,994,997, 6,108,212, 6,130,601, 6,171,921, 6,194,990, 6,225,035, 6,229,098, 6,232,042 and 6,256,866, each of which is incorporated, by reference, in its entirety in this application.
Some of the drawbacks of the prior art methods, formulations and resistors are i) the wide ranges of variability of the resistance of the cured resistors, thus the wide ranges of variability of current flowing through the resistors and consequently lower reliability of circuit performance, ii) the substantial printed resistor dimensional changes during the curing process and iii) the high percentage of volatile solvents contained in the ink and paste formulations of the prior art.
The present invention overcomes the drawbacks of the prior art by providing novel polymer thick-film resistive paste formulations which have lower percentages of volatiles. It also overcomes the problems associated with dimensional changes by providing pastes with high thixotropic index, a novel apparatus for application of polymer-thick film resistive pastes and a novel method of applying polymer thick-film resistive pastes which increases the shear strain rate in the paste bead prior to its transfer to the cavity where the resistor is to be formed.